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1Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
Carnegie Mellon
Machine-Level Programming II: Control
CS140 – Computer Organization and Assembly
Carnegie Mellon
Slides Courtesy of:
Randal E. Bryant and David R. O’Hallaron
2Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
First…
https://www.youtube.com/watch?v=IvUU8joBb1Q
3Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
Carnegie Mellon
Today
Control: Condition codes
Conditional branches
Loops
Switch Statements
4Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
Carnegie Mellon
Processor State (x86-64, Partial)
Information about currently executing program
Temporary data( %rax, … )
Location of runtime stack( %rsp )
Location of current code control point( %rip, … )
Status of recent tests( CF, ZF, SF, OF )
%rip
Registers
Current stack topInstruction pointer
CF ZF SF OF Condition codes
%rsp
%r8
%r9
%r10
%r11
%r12
%r13
%r14
%r15
%rax
%rbx
%rcx
%rdx
%rsi
%rdi
%rbp
5Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
Carnegie Mellon
Condition Codes (Implicit Setting)
Single bit registers
CF Carry Flag (for unsigned) SF Sign Flag (for signed)
ZF Zero Flag OF Overflow Flag (for signed)
Implicitly set (think of it as side effect) by arithmetic operationsExample: addq Src,Dest ↔ t = a+b
CF set if carry out from most significant bit (unsigned overflow)
ZF set if t == 0
SF set if t < 0 (as signed)
OF set if two’s-complement (signed) overflow(a>0 && b>0 && t<0) || (a<0 && b<0 && t>=0)
Not set by leaq instruction
6Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
Carnegie Mellon
Condition Codes (Explicit Setting: Compare)
Explicit Setting by Compare Instructioncmpq Src2, Src1
cmpq b,a like computing a-b without setting destination
CF set if carry out from most significant bit (used for unsigned comparisons)
ZF set if a == b
SF set if (a-b) < 0 (as signed)
OF set if two’s-complement (signed) overflow(a>0 && b<0 && (a-b)<0) || (a<0 && b>0 && (a-b)>0)
7Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
Carnegie Mellon
Condition Codes (Explicit Setting: Test)
Explicit Setting by Test instructiontestq Src2, Src1
testq b,a like computing a&b without setting destination
Sets condition codes based on value of Src1 & Src2
Useful to have one of the operands be a mask
ZF set when a&b == 0
SF set when a&b < 0
8Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
Carnegie Mellon
Reading Condition Codes
SetX Instructions
Set low-order byte of destination to 0 or 1 based on combinations of condition codes
Does not alter remaining 7 bytes
SetX Condition Description
sete ZF Equal / Zero
setne ~ZF Not Equal / Not Zero
sets SF Negative
setns ~SF Nonnegative
setg ~(SF^OF)&~ZF Greater (Signed)
setge ~(SF^OF) Greater or Equal (Signed)
setl (SF^OF) Less (Signed)
setle (SF^OF)|ZF Less or Equal (Signed)
seta ~CF&~ZF Above (unsigned)
setb CF Below (unsigned)
9Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
%rsp
x86-64 Integer Registers
Can reference low-order byte
%al
%bl
%cl
%dl
%sil
%dil
%spl
%bpl
%r8b
%r9b
%r10b
%r11b
%r12b
%r13b
%r14b
%r15b
%r8
%r9
%r10
%r11
%r12
%r13
%r14
%r15
%rax
%rbx
%rcx
%rdx
%rsi
%rdi
%rbp
10Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
cmpq %rsi, %rdi # Compare x:y
setg %al # Set when >
movzbl %al, %eax # Zero rest of %rax
ret
Carnegie Mellon
Reading Condition Codes (Cont.) SetX Instructions:
Set single byte based on combination of condition codes
One of addressable byte registers Does not alter remaining bytes
Typically use movzbl to finish job
32-bit instructions also set upper 32 bits to 0
int gt (long x, long y)
{
return x > y;
}
Register Use(s)
%rdi Argument x
%rsi Argument y
%rax Return value
11Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
Carnegie Mellon
Today
Control: Condition codes
Conditional branches
Loops
Switch Statements
12Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
Carnegie Mellon
Jumping
jX Instructions
Jump to different part of code depending on condition codes
jX Condition Description
jmp 1 Unconditional
je ZF Equal / Zero
jne ~ZF Not Equal / Not Zero
js SF Negative
jns ~SF Nonnegative
jg ~(SF^OF)&~ZF Greater (Signed)
jge ~(SF^OF) Greater or Equal (Signed)
jl (SF^OF) Less (Signed)
jle (SF^OF)|ZF Less or Equal (Signed)
ja ~CF&~ZF Above (unsigned)
jb CF Below (unsigned)
13Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
Carnegie Mellon
Conditional Branch Example (Old Style)
long absdiff
(long x, long y)
{
long result;
if (x > y)
result = x-y;
else
result = y-x;
return result;
}
absdiff:
cmpq %rsi, %rdi # x:y
jle .L4
movq %rdi, %rax
subq %rsi, %rax
ret
.L4: # x <= y
movq %rsi, %rax
subq %rdi, %rax
ret
Generationcsgateway> gcc –O1 -S –fno-if-conversion control.c
Register Use(s)
%rdi Argument x
%rsi Argument y
%rax Return value
14Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
Carnegie Mellon
Expressing with Goto Code
long absdiff
(long x, long y)
{
long result;
if (x > y)
result = x-y;
else
result = y-x;
return result;
}
C allows goto statement
Jump to position designated by label
long absdiff_j
(long x, long y)
{
long result;
int ntest = x <= y;
if (ntest) goto Else;
result = x-y;
goto Done;
Else:
result = y-x;
Done:
return result;
}
15Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
Carnegie Mellon
C Code
val = Test ? Then_Expr : Else_Expr;
Goto Versionntest = !Test;if (ntest) goto Else;
val = Then_Expr;goto Done;
Else:
val = Else_Expr;Done:
. . .
General Conditional Expression Translation (Using Branches)
Create separate code regions for then & else expressions
Execute appropriate one
val = x>y ? x-y : y-x;
16Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
Carnegie Mellon
C Code
val = Test? Then_Expr: Else_Expr;
Goto Version
result = Then_Expr;eval = Else_Expr;nt = !Test;if (nt) result = eval;
return result;
Using Conditional Moves Conditional Move Instructions
Instruction supports:
if (Test) Dest Src
Supported in post-1995 x86 processors
GCC tries to use them
But, only when known to be safe
Why? Branches are very disruptive to
instruction flow through pipelines
Conditional moves do not require control transfer
17Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
Carnegie Mellon
Conditional Move Example
absdiff:
movq %rdi, %rax # x
subq %rsi, %rax # result = x-y
movq %rsi, %rdx
subq %rdi, %rdx # eval = y-x
cmpq %rsi, %rdi # x:y
cmovle %rdx, %rax # if <=, result = eval
ret
long absdiff
(long x, long y)
{
long result;
if (x > y)
result = x-y;
else
result = y-x;
return result;
}
Register Use(s)
%rdi Argument x
%rsi Argument y
%rax Return value
18Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
Carnegie Mellon
Expensive Computations
Bad Cases for Conditional Move
Both values get computed
Only makes sense when computations are very simple
val = Test(x) ? Hard1(x) : Hard2(x);
Risky Computations
Both values get computed
May have undesirable effects
val = p ? *p : 0;
Computations with side effects
Both values get computed
Must be side-effect free
val = x > 0 ? x*=7 : x+=3;
19Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
Carnegie Mellon
Today
Control: Condition codes
Conditional branches
Loops
Switch Statements
20Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
Carnegie Mellon
C Codelong pcount_do
(unsigned long x) {
long result = 0;
do {
result += x & 0x1;
x >>= 1;
} while (x);
return result;
}
Goto Versionlong pcount_goto
(unsigned long x) {
long result = 0;
loop:
result += x & 0x1;
x >>= 1;
if(x) goto loop;
return result;
}
“Do-While” Loop Example
Count number of 1’s in argument x (“popcount”)
Use conditional branch to either continue looping or to exit loop
21Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
Carnegie Mellon
Goto Version
“Do-While” Loop Compilation
movl $0, %eax # result = 0
.L2: # loop:
movq %rdi, %rdx
andl $1, %edx # t = x & 0x1
addq %rdx, %rax # result += t
shrq %rdi # x >>= 1
jne .L2 # if (x) goto loop
rep; ret
long pcount_goto
(unsigned long x) {
long result = 0;
loop:
result += x & 0x1;
x >>= 1;
if(x) goto loop;
return result;
}
Register Use(s)
%rdi Argument x
%rax result
22Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
Carnegie Mellon
C Code
do
Bodywhile (Test);
Goto Version
loop:
Bodyif (Test)
goto loop
General “Do-While” Translation
Body: {
Statement1;
Statement2;
…
Statementn;
}
23Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
Carnegie Mellon
While version
while (Test)Body
General “While” Translation #1
“Jump-to-middle” translation
Used with -Og Goto Version
goto test;
loop:
Bodytest:
if (Test)goto loop;
done:
24Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
Carnegie Mellon
C Codelong pcount_while
(unsigned long x) {
long result = 0;
while (x) {
result += x & 0x1;
x >>= 1;
}
return result;
}
Jump to Middle Versionlong pcount_goto_jtm
(unsigned long x) {
long result = 0;
goto test;
loop:
result += x & 0x1;
x >>= 1;
test:
if(x) goto loop;
return result;
}
While Loop Example #1
Compare to do-while version of function
Initial goto starts loop at test
25Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
Carnegie Mellon
While version
while (Test)Body
Do-While Version
if (!Test) goto done;
do
Bodywhile(Test);
done:
General “While” Translation #2
“Do-while” conversion
Used with –O1
Goto Version
if (!Test)goto done;
loop:
Bodyif (Test)
goto loop;
done:
26Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
Carnegie Mellon
C Codelong pcount_while
(unsigned long x) {
long result = 0;
while (x) {
result += x & 0x1;
x >>= 1;
}
return result;
}
Do-While Versionlong pcount_goto_dw
(unsigned long x) {
long result = 0;
if (!x) goto done;
loop:
result += x & 0x1;
x >>= 1;
if(x) goto loop;
done:
return result;
}
While Loop Example #2
Compare to do-while version of function
Initial conditional guards entrance to loop
27Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
Carnegie Mellon
“For” Loop Form
for (Init; Test; Update )
Body
General Form
#define WSIZE 8*sizeof(int)
long pcount_for
(unsigned long x)
{
size_t i;
long result = 0;
for (i = 0; i < WSIZE; i++)
{
unsigned bit =
(x >> i) & 0x1;
result += bit;
}
return result;
}
i = 0
i < WSIZE
i++
{
unsigned bit =
(x >> i) & 0x1;
result += bit;
}
Init
Test
Update
Body
28Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
Carnegie Mellon
“For” Loop While Loop
for (Init; Test; Update )
Body
For Version
Init;
while (Test ) {
Body
Update;
}
While Version
29Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
Carnegie Mellon
For-While Conversionlong pcount_for_while
(unsigned long x)
{
size_t i;
long result = 0;
i = 0;
while (i < WSIZE)
{
unsigned bit =
(x >> i) & 0x1;
result += bit;
i++;
}
return result;
}
i = 0
i < WSIZE
i++
{
unsigned bit =
(x >> i) & 0x1;
result += bit;
}
Init
Test
Update
Body
30Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
Carnegie Mellon
C Code
“For” Loop Do-While Conversion
Initial test can be optimized away
long pcount_for
(unsigned long x)
{
size_t i;
long result = 0;
for (i = 0; i < WSIZE; i++)
{
unsigned bit =
(x >> i) & 0x1;
result += bit;
}
return result;
}
Goto Versionlong pcount_for_goto_dw
(unsigned long x) {
size_t i;
long result = 0;
i = 0;
if (!(i < WSIZE))
goto done;
loop:
{
unsigned bit =
(x >> i) & 0x1;
result += bit;
}
i++;
if (i < WSIZE)
goto loop;
done:
return result;
}
Init
!Test
Body
Update
Test
31Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
Carnegie Mellon
Today
Control: Condition codes
Conditional branches
Loops
Switch Statements
32Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
Carnegie Mellon
Switch Statement Example
Multiple case labels
Here: 5 & 6
Fall through cases Here: 2
Missing cases
Here: 4
long switch_eg
(long x, long y, long z)
{
long w = 1;
switch(x) {
case 1:
w = y*z;
break;
case 2:
w = y/z;
/* Fall Through */
case 3:
w += z;
break;
case 5:
case 6:
w -= z;
break;
default:
w = 2;
}
return w;
}
33Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
Carnegie Mellon
Jump Table Structure
Code Block0
Targ0:
Code Block1
Targ1:
Code Block2
Targ2:
Code Blockn–1
Targn-1:
•
•
•
Targ0
Targ1
Targ2
Targn-1
•
•
•
jtab:
goto *JTab[x];
switch(x) {
case val_0:
Block 0case val_1:
Block 1• • •
case val_n-1:
Block n–1}
Switch Form
Translation (Extended C)
Jump Table Jump Targets
34Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
Carnegie Mellon
Switch Statement Example
Setup:
long switch_eg(long x, long y, long z)
{
long w = 1;
switch(x) {
. . .
}
return w;
}
switch_eg:
movq %rdx, %rcx
cmpq $6, %rdi # x:6
ja .L8
jmp *.L4(,%rdi,8)
What range of values takes default?
Note that w not initialized here
Register Use(s)
%rdi Argument x
%rsi Argument y
%rdx Argument z
%rax Return value
35Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
Carnegie Mellon
Switch Statement Example
long switch_eg(long x, long y, long z)
{
long w = 1;
switch(x) {
. . .
}
return w;
}
Indirect jump
Jump table.section .rodata
.align 8
.L4:
.quad .L8 # x = 0
.quad .L3 # x = 1
.quad .L5 # x = 2
.quad .L9 # x = 3
.quad .L8 # x = 4
.quad .L7 # x = 5
.quad .L7 # x = 6
Setup:
switch_eg:
movq %rdx, %rcx
cmpq $6, %rdi # x:6
ja .L8 # Use default
jmp *.L4(,%rdi,8) # goto *JTab[x]
36Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
Carnegie Mellon
Assembly Setup Explanation
Table Structure
Each target requires 8 bytes
Base address at .L4
Jumping Direct: jmp .L8
Jump target is denoted by label .L8
Indirect: jmp *.L4(,%rdi,8)
Start of jump table: .L4
Must scale by factor of 8 (addresses are 8 bytes)
Fetch target from effective Address .L4 + x*8
Only for 0 ≤ x ≤ 6
Jump table
.section .rodata
.align 8
.L4:
.quad .L8 # x = 0
.quad .L3 # x = 1
.quad .L5 # x = 2
.quad .L9 # x = 3
.quad .L8 # x = 4
.quad .L7 # x = 5
.quad .L7 # x = 6
37Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
.section .rodata
.align 8
.L4:
.quad .L8 # x = 0
.quad .L3 # x = 1
.quad .L5 # x = 2
.quad .L9 # x = 3
.quad .L8 # x = 4
.quad .L7 # x = 5
.quad .L7 # x = 6
Carnegie Mellon
Jump Table
Jump tableswitch(x) {
case 1: // .L3
w = y*z;
break;
case 2: // .L5
w = y/z;
/* Fall Through */
case 3: // .L9
w += z;
break;
case 5:
case 6: // .L7
w -= z;
break;
default: // .L8
w = 2;
}
38Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
Carnegie Mellon
Code Blocks (x == 1).L3:
movq %rsi, %rax # y
imulq %rdx, %rax # y*z
ret
switch(x) {
case 1: // .L3
w = y*z;
break;
. . .
}
Register Use(s)
%rdi Argument x
%rsi Argument y
%rdx Argument z
%rax Return value
39Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
Carnegie Mellon
Handling Fall-Through
long w = 1;
. . .
switch(x) {
. . .
case 2:
w = y/z;
/* Fall Through */
case 3:
w += z;
break;
. . .
}case 3:
w = 1;
case 2:
w = y/z;
goto merge;
merge:
w += z;
40Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
Carnegie Mellon
Code Blocks (x == 2, x == 3).L5: # Case 2
movq %rsi, %rax
cqto
idivq %rcx # y/z
jmp .L6 # goto merge
.L9: # Case 3
movl $1, %eax # w = 1
.L6: # merge:
addq %rcx, %rax # w += z
ret
long w = 1;
. . .
switch(x) {
. . .
case 2:
w = y/z;
/* Fall Through */
case 3:
w += z;
break;
. . .
} Register Use(s)
%rdi Argument x
%rsi Argument y
%rdx Argument z
%rax Return value
41Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
Carnegie Mellon
Code Blocks (x == 5, x == 6, default).L7: # Case 5,6
movl $1, %eax # w = 1
subq %rdx, %rax # w -= z
ret
.L8: # Default:
movl $2, %eax # 2
ret
switch(x) {
. . .
case 5: // .L7
case 6: // .L7
w -= z;
break;
default: // .L8
w = 2;
}
Register Use(s)
%rdi Argument x
%rsi Argument y
%rdx Argument z
%rax Return value
42Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
Carnegie Mellon
Summarizing
C Control
if-then-else
do-while
while, for
switch
Assembler Control Conditional jump
Conditional move
Indirect jump (via jump tables)
Compiler generates code sequence to implement more complex control
Standard Techniques Loops converted to do-while or jump-to-middle form
Large switch statements use jump tables
Sparse switch statements may use decision trees (if-elseif-elseif-else)
43Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
Carnegie Mellon
Summary
Today
Control: Condition codes
Conditional branches & conditional moves
Loops
Switch statements
Next Time Stack
Call / return
Procedure call discipline
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